The present invention relates to a reactor for performing endothermic catalytic reactions. An endothermic catalytic reaction mentioned by way of example is the reaction of methane and ammonia to give hydrocyanic acid in the presence of a catalyst.
The catalytic reaction of methane and ammonia to give hydrocyanic acid is described in Ullmann""s Encyclopedia of Industrial Chemistry, 5th ed. (1987), vol. A8, pages 162-163. Reaction takes place in the presence of an appropriate catalyst at temperatures of about 1000 to 1350xc2x0 C. The reaction may be performed, for example, in tube bundle reactors. The reaction tubes essentially consist of aluminum oxide and the internal surfaces are provided with a catalytically active coating which contains mostly platinum. The reaction tubes are suspended in the combustion chamber and are externally heated to the reaction temperature by combustion of a fuel gas, usually methane or natural gas, in an oxygen-containing gas which proceeds in a combustion chamber. The reaction tubes are typically about 2 m long and have an internal diameter of 16 to 18 mm.
The energy consumption required to produce one kilogram of desired product is critical to the economic viability of this type of process. In the case of the tube bundle reactor described above, about 40 MJ are required to produce one kilogram of hydrocyanic acid.
In European patent application EP 0752390A1, which is not a prior publication, a process for preparing hydrocyanic acid is described in which a monolithic arrangement of heating and reaction channels is used instead of the tube bundle reactor. The internal wall surfaces of the reaction channels are coated with a catalyst and the reactants flow through these reaction channels.
The reaction temperature required is produced, in this case, by combustion of a fuel gas in the heating channels. For this purpose, a mixture of fuel gas and combustion air is passed through the heating channels countercurrent to the flow of the reactants. On entering the heating channels, the gas mixture is ignited by electrical heating wires which are inserted into the heating channels.
The combination of heating and reaction channels in a monolithic arrangement can almost halve the energy demand required per kilogram of hydrocyanic acid produced as compared with a tube bundle reactor. In addition, the space-time yield can be greatly increased.
Despite these advantageous characteristics of the monolithic counterflow reactor, further improvements are required because combustion of the fuel gas/air mixture in the heating channels is not sufficiently stable.
In German patent application DE 34 02 713 A1 there is described a countercurrent reactor formed of a multiplicity of parallel plates for carrying out endothermic reactions with reaction temperatures up to 800xc2x0 C. The endothermic reaction takes place in a first gaseous mixture, while the heat for reaction is supplied by way of an exothermic reaction in a second gaseous mixture. The first gaseous mixture and the second gaseous mixture are sent through the reactor in countercurrent direction with respect to each other whereby the first and second gaseous mixture stream through the space between the plates of the plate reactor in an alternating manner. The space between the plate is filled with particulate catalyst which catalyzes the particular endothermic or exothermic reaction.
German patent application 42 14 479 A1 discloses in FIG. 4, inter alia, a countercurrent reactor which can be used for strong exothermic or strong endothermic reactions. The reactor is made up of a plurality of mutually parallel arranged corrugated iron plates. The spaces between the plates form an alternating spatial arrangement for a throughput of a reaction gas mixture for an endothermic reaction and a heating gas. Instead of filling the spaces between the plates with a particulate catalyst, DE 42 14 579 A1 utilizes a catalyst for the endothermic reaction which is deposited as a layer on the surface of the plates which are in contact with the reactants. The surface of the corrugated iron plates which are in the path of the heating gas are not coated with a catalyst. The necessary energy for the endothermic reaction is conveyed to the reactor from outside through a hot heating gas.
A disadvantage of both of these prior reactors is the fact that they are suited for operation only at relatively low temperatures. At reaction temperatures over 800xc2x0 C. and in particular at the reaction temperatures for the preparation of hydrocyanic acid from methane and ammonia according to the BMA process at 1000 to 1400xc2x0 C. there is a tendency for these reactors to undergo a oxidation of the metallic plates from which they are constructed.
An object of the present invention is therefore to provide a monolithic reactor for performing endothermic catalytic reactions at temperatures above 800xc2x0 C. which can withstand these elevated temperatures and which has improved and stable combustion in the heating channels.
The above and other objects of the invention are achieved by a reactor for performing endothermic catalytic reactions which contains an arrangement of a plurality of heating and reaction channels running parallel to each other, whose walls consist of a heat-resistant and gas-impermeable material, wherein the reaction channels contain a catalyst for the endothermic catalytic reaction and the heating channels contain a catalyst for the catalytic combustion of a fuel gas/air mixture. The reaction channels and the heating channels have a length L.
The reactor is of a structure such that the reaction channels and the heating channels are arranged in a monolithic block, wherein the gas impervious material is a ceramic and the catalyst for the catalytic reaction and the catalyst for the catalytic combustion can be in the form of a layer or coating upon the wall surface of the reaction and heating channels.
In the reactor according to the present invention, heating and reaction channels are directly adjacent. In order to prevent passage of the gases through the walls of the reaction channels into the heating channels and vice versa, the reactor is constructed from a gas-tight ceramic which is also resistant to the high reaction temperatures required. This type of ceramic has a specific surface area which is only marginally greater than the geometric surface area of the reactor.
In contrast to the known monolithic reactor from EP 0752390A1, combustion in the heating channels of the reactor of the present invention is not initiated electrically, but instant catalytic combustion of the fuel gas/air mixture takes place. The reactor according to the invention therefore has two catalysts with different functions, a xe2x80x9creaction catalystxe2x80x9d deposited in the reaction channels for catalyzing the desired endothermic reaction between the reactants and a xe2x80x9ccombustion catalystxe2x80x9d deposited on the walls of the heating channels for combustion of the fuel gas/air mixture.